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Lasers & Sources

Nonlinear photonic crystals for passive switches

A layered photonic crystal structure enhances nonlinear optical properties of a dopant dye.
22 May 2008, SPIE Newsroom. DOI: 10.1117/2.1200805.1144

Passive optical components that are controlled solely by incident light and not by any external (e.g., electrical or mechanical) control are highly desirable for many applications. These include switching and sensor protection materials, such as thin film coatings that could be applied to glasses or windows to block harmful radiation over a broad spectrum. The ability of light to control the reflection, refraction, or transmission (or, typically, all three) of the beam of light itself or other beams in a material depends on the nonlinear optical properties of the material and their type of nonlinearity. Materials with large nonlinearities need to be developed in order to realize the full potential of such devices.

Until now, research has focused primarily on synthesizing new materials that have superior nonlinear optical performance compared to current materials.1,2 A complementary second approach is to incorporate existing nonlinear materials into structures that enhance their nonlinear properties.3,4 We have incorporated a nonlinear dye into a 1D photonic crystal and used an intense light `pump’ beam to induce nonlinear behavior, then monitored the transmission of a second probe beam at different arrival times. We have shown that the nonlinear optical properties of the structure are greater than that of a comparable bulk (unlayered) structure and the dynamics of the dye are not adversely affected by the layered structure.

Nonlinear photonic crystals have been extensively studied theoretically,5,6,7 but less has been done experimentally4,5,6,7,8 in part due to fabrication issues. Due to the small induced refractive index change in many nonlinear materials, many layers are required to produce a measurable effect. We used the forced assembly coextrusion process,9,10 with which films can be produced with up to 4096 layers, each with a thickness on the order of a quarter-wavelength of light. A variety of polymers are amenable to this process as are organic and inorganic dopants, which allows for great flexibility in structure design.

Figure 1. A 1D nonlinear photonic crystal. (a) Under low intensity excitation the transmission is proportional to the linear absorption coefficient (α) of the dyed layers (green). (b) Under high intensity excitation, the transmission depends not only on the nonlinear absorption (αNL) of the material but also the induced reflectivity (RNL). (Click to enlarge image.)

The particular system we examined is a 1D photonic crystal composed of 256 alternating layers of polycarbonate (PC) doped with a high concentration of the organic dye lead(II)tetrakis(4-cumylphenoxy)-phthalocyanine (PbPc(CP)4) and undyed layers of polyethylene-terephthalate (PET) resulting in a total of 512 layers. Under low light excitation the PET and doped-PC layers are nearly index matched, but under high excitation a refractive index mismatch is induced. The nonlinear refractive index is small, but under the correct structure configuration in which a large portion of the layers produce constructive interference, the induced photonic band gap can cause a large increase in reflectivity, as illustrated in Figure 1.

Figure 2. Formation of nonlinear band gap as a function of time delay at an excitation density of 4%. Percentage change due to bandgap is calculated by dividing an effective nonlinear absorption cross-section for the photonic crystal results by that of the nonlinear absorption cross-section of the bulk material. (Top) Before zero delay: - 30ps, (middle) at a time delay of 1.1ns, (bottom) at a time delay of 2.4ns.

We used PbPc(CP)4 because it has a strong and broad excited-state absorption band in the visible coupled with a long excited-state lifetime (>10ns). Polycarbonate was chosen as the host polymer due to its optical clarity and the ability to dope the polymer with up to 20% by weight of PbPc(CP)4 with limited dye aggregation. This system is designed for sensor protection applications, but if we were to replace the PbPc(CP)4 dye with a material that had low nonlinear absorption (but large nonlinear refraction) and a short lifetime then this structure would operate as an optical switch.

We determined the nonlinear characteristics of the PbPc(CP)4 photonic crystal with a series of femtosecond white-light continuum (WLC) transient absorption experiments. For these measurements, the output of a Ti:sapphire regenerative amplifier (771nm, 1kHz repetition rate, 150fs pulses from a Clark MXR CPA-2001) was used to excite the sample and also to produce a WLC in sapphire to probe the sample at various time delays from -100ps to 2.4ns after excitation. We directly compared a bulk sample with an identical nonlinear optical path length to that of the nonlinear photonic crystal structure. The results from these experiments can be seen in Figure 2, in which we have divided an effective nonlinear absorption cross-section for the photonic crystal results by that of the nonlinear absorption cross-section of the bulk material to arrive at a percentage change due to the bandgap. The results show three things: first, an enhancement over the bulk structure is observed; secondly, the structure has a very fast rise time (<100ps); and finally, the enhancement remains at the maximum time delay we could measure.

To our knowledge we are the first to demonstrate experimentally the ability to induce a nonlinear photonic band gap in a layered dielectric polymer structure. The induced reflectivity of the photonic crystal adds to the inherent nonlinear absorption over a 50nm bandwidth. The additional decrease in transmission due to the observed reflectivity change is about 8% of that due to the inherent nonlinear absorption. We are currently pursuing several avenues to increase the magnitude of the effect.

We thank Professor Eric Baer and Aditya Ranade of Case Western Reserve University for the fabrication of the photonic crystal. RSL acknowledges the National Research Council for a postdoctoral fellowship that he held while conducting this research at the Naval Research Laboratory. The work was supported by the Office of Naval Research.

Richard Lepkowicz
Rose-Hulman Institute of Technology
Terre Haute, IN

Richard S. Lepkowicz is an assistant professor of physics and optical engineering at Rose-Hulman Institute of Technology. He received his PhD in optics from the University of Central Florida in 2004, and completed a National Research Council Postdoctoral Fellowship at the Naval Research Laboratory from 2004 – 2006. His research interests include nonlinear spectroscopy, nonlinear optical structures, and optical design and fabrication using novel polymer optical components.

Steven Flom, James Shirk
Optical Sciences Division
Naval Research Laboratory
Washington, DC

Steven Flom received his PhD in chemistry in 1985 from the University of Minnesota. After post-doctoral studies at Syracuse University, the University of Minnesota, and Honeywell, he joined the Optical Sciences Division at the Naval Research Lab in 1991. He is an author on more than 70 publications and five patents. His research interests include optical materials, ultrafast spectroscopy and nonlinear optics.

James S. Shirk received his PhD in chemistry from the University of California, Berkeley. He was professor of chemistry at the Illinois Institute of Technology before coming to the Naval Research Laboratory in 1987. His research interests involve the development of organic optical materials including hierarchically structured linear and nonlinear polymers.